GF - I 2
Main Limit Switch
Introduction
Figure 1
The main limit switch (Figure 1) is a normally closed
temperature activated switch that will open to shut down
burner operation if the furnace discharge air tem-
perature exceeds switch open temperature. This
switch keeps the furnace operating safely by prevent-
ing significantly increased air temperature to occur
due to a lack of indoor blower air volume.
The switch is located on the supply air side of the
furnace (Figure 2.) The switch is an auto reset type.
The open temperature setting of the switch can be
found on the furnace data plate (Figure 3.) This switch
is not a one-replaces-all part, as different furnace mod-
els will have unique switch temperature ratings. If the
switch must be replaced, an exact replacement must
be used.
Figure 2
Figure 3
If the main limit switch re-closes at any time, the fur-
nace will then retry an attempt at normal furnace op-
eration.
Electrical circuit
Figure 4
During a call for heat, the furnace control board moni-
tors the limit switch circuit to ensure all switches re-
main closed during furnace operation. Should one of
the switches open, due to high temperature detection,
the furnace control will immediately de-energize the
gas valve to shut down burner operation. The furnace
control board will then turn on both the induced draft
blower and indoor air blower in an attempt to cool down
the furnace.
Main limit switch
Main air limit switch
on furnace
The main limit is wired into the 24 volt limit switch
circuit (Figure 4). The limit switch circuit includes
both the main limit and burner limits. All of these
switches are normally closed devices. The limit switch
circuit is connected to the furnace control board via
the 15 pin plug that is located on the furnace control
board.
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